“IBM has released the Web Services Tool Kit for Mobile Devices. Its a free set of tools that provide run-time environments that allow development of applications that use Web Services on small mobile devices. This tool kit’s Java Web service run-time environment is supported on PoctketPC, Palm, and BlackBerry.” More at the IBM Website
Which is older the OS/2 still has some forms of support (you still can download stuff from Apple’s FTP site).
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http://homepage.mac.com/softkid/
I’m still waiting for an open source and open hardware cellphone running Linux or BSD
Seriously though, it would be pretty cool. I’m surprised nobody has actually tried to construct one.
Dynapad has had an Open Source Webservices toolkit for a while. A PDA running Dynapad can both run a web services server as well as access them using XML-RPC, SOAP or some other custom HTTP-based protocol.